The speed and completeness of chemical reactions depend to a large extent on whether and how far it is possible to mix the reaction partners with one another. Optimal reaction conditions arise if the reaction partners form a homogeneous mixture, so that the diffusion paths are as short as possible. This ideal condition can usually not be obtained at tolerable expense in pipe lines, and particularly in flue gas passages from large boilers, with a cross-section of several m.sup.2, for example 3 m.sup.2 to 50 m.sup.2, in which an auxiliary stream is supplied to a main stream at right angles to the direction of flow so that it normally penetrates to the centre of the main stream. Known solutions are pipe registers installed in the flow or injection nozzles installed in the walls, but all of these lead to a concentration profile across the pipe cross-section, with a concentration peak in the centre of the pipe, which is more or less pronounced depending on the injection conditions.
Relatively good mixing can be obtained with the aid of injectors where one mixing partner is injected through a central nozzle and the other through the annular gap between the central nozzle and the pipe surrounding the nozzle concentrically. A disadvantage of the use of such injectors is however that they require corresponding internal fittings in the region of the mixing path, which leads to difficulties, especially with hot and/or corrosive and dust-containing gases, and is associated with a considerable pressure loss. This, for example, is the case with flue gases from industrial furnaces, whose temperatures lie between 800.degree. to 1200.degree. C. and which contain corrosive components such as sulphur, fluorine and chlorine, and in solid fuel furnaces, for example coal dust furnaces, whose flue gases are loaded with substantial amounts of dust. However, it is precisely these pollutant-containing gases for which subsequent treatment to remove the pollutants is imperative.
Thus, in view of the legal requirements, the flue gases from stationary industrial furnace plants as a rule require secondary measures for removing the nitrogen oxides contained in the flue gases to be employed even if primary measures are employed to reduce the amount of nitrogen oxides formed. A number of wet processes, namely oxidation or reduction-absorption processes or oxidation/reduction processes, and also dry processes for catalytic or non-catalytic reduction of the nitrogen oxides, are known for this purpose. The wet processes and the catalytic processes are however very troublesome and costly as a result of the solid addition materials or catalysts required to remove the nitrogen oxides. This does not apply to the so-called progressive combustion process, in which the fuel supply is divided between at least two combustion stages, followed by a burn-out stage. The technical application of this process therefore involves at least three stages, of which the first stage should be operated super-stoichiometrically, i.e. with an excess of air, the second stage sub-stoichiometrically, i.e. with a deficiency of air, and the third stage again super-stoichiometrically. The second fuel-rich stage can be considered as an NO-reduction stage in which the resulting carbon monoxide reacts with the nitric oxide to form the harmless products carbon dioxide and nitrogen.
The degree of non-catalytic reduction of the nitrogen oxides with the aid of a fluid reducing agent depends in all processes essentially on how far and how quickly the reducing agent can be mixed homogeneously with the usually quickly-flowing flue gases in a zone that is as short as possible, without too much outlay on apparatus and/or too high operating costs.